In conventional level pour or hot top casting, molten metal is poured into the feed end of an open-ended tubular mold and solidified or partially solidified metal exits from the discharge end of the mold. The mold itself is cooled by a body of coolant maintained at the backside of the mold by means of a water jacket. Coolant, usually water or water fortified with dissolved gas, is applied around the periphery of the ingot as it exits from the mold to effect solidification. In the casting of light metals, such as aluminum, coolant is usually directed by means of one or more baffles from the body of coolant in the water jacket down the backside of the mold and out suitable slots or conduits at the bottom of the mold onto the ingot exiting the discharge end of the mold.
Electromagnetic (EM) casting is similar to the above-described conventional level pour or hot top casting except that the lateral shape of the molten metal is controlled by electromagnetic pressure generated by the annular inductor surrounding the column of molten metal, rather that the bore of the mold as in conventional level pour or hot top casting.
In vertical level pour or hot top casting and EM casting, a bottom block is positioned within the discharge end of the mold (for level pour or hot top casting) or within the discharge end of the electromagnetic inductor (for EM casting) to close off the discharge opening and to hold the molten metal until it has solidified enough to maintain its final desired shape. When the metal has been sufficiently solidified, the bottom block is lowered out of the discharge end of the mold or inductor to allow the solidified ingot to be discharged from the mold or inductor in a continuous or semi-continuous fashion. Once the withdrawal of the bottom block begins, the drop rate thereof is usually maintained at a constant level until the end of the cast, because any sudden change in the drop rate can result in changes in the cross-sectional dimensions of the solidified ingot along the length thereof and can cause serious surface defects on the ingot.
In conventional level pour or hot top casting, there is very little, and in EM casting, there is essentially no horizontal support of the solidified ingot in its downward descent, so the ingot must be well balanced on the bottom block to avoid rocking or leaning off center. However, as the butt of the ingot solidifies and cools, the ingot shrinks. The bottom face of the forming ingot in contact with the bottom block begins to curl away from the surface of the bottom block as the metal begins to solidify and contract. Frequently, as the butt end of the ingot begins to curl away from the top of the bottom block, the forming ingot shell will not be sufficiently strong to support itself and one side of the ingot will start to collapse and a crack may form at the stress point at the edge of the butt which can ultimately extend the entire length of the ingot and thereby require its scrapping.
The formation of dish-shaped butts is a significant problem in casting with bottom blocks, especially in casting alloys having an intermediate size melting range (e.g., 35.degree.-200.degree. F., particularly 40.degree.-140.degree. F.). With relatively pure alloys, such as 1100 (Aluminum Association alloy designation), the melting range is so narrow that rapid solidification of the butt is assured under normal casting conditions, thereby minimizing the chances of forming a dish-shaped butt. On the other hand, with highly alloyed materials, even though the temperature range between the solidus and liquidus points is broad, the strength of the forming ingot due to the alloying constituents is sufficiently high to preclude the formation of dish-shaped butts.
A typical prior art bottom block are shown in U.S. Pat. Nos. 3,948,310, 4,509,580 and 4987,950.
Accordingly, it would be advantageous to provide an economical and effective bottom block for casting and method of casting metal that results in less residual stress and cracking in the ingot.
The primary object of the present invention is to provide a method and bottom block for casting metal that results in less residual stress and cracking in the ingot.
Another object of the present invention is to provide a method and bottom block for casting metal that results in less residual stress and cracking in the ingot without water cooling the bottom block.
These and other objects and advantages of the present invention will be more fully understood and appreciated with reference to the following description